Target electrode assembly



Jan. 26", 1960 H. J. HANNAM 2,922,907

TARGET ELECTRODE ASSEMBLY Filed May 23, 1958 2 Sheets-Sheet 1 .INVENTOR:HERBERT J. HANNAM,

ms ATTORNEY.

Jan. 26, 1960 H. J. HANNAM TARGET ELECTRODE ASSEMBLY 2 Sheets-Sheet 2Filed May 23, 1958 FIG.4.

FIG.3.

M A N R .TJ MT m WB R E H HIS ATTORNEY.

2,922,907 TARGET ELECTRODE ASSEMBLY Herbert James Hannam, Altamont,N.Y., assignor to General Electric Company, a corporation of New YorkApplication May 23, 1958, Serial No. 737,348

6 Claims. (Cl. 313-68) My invention relates to an improved targetelectrode assembly and more particularly to an improved assembly of thistype for producing a point-by-point electric charge patterncorresponding to a visual image or other information to be converted toelectrical signals by scanning the target electrode with an electronbeam. Additionally, my invention relates to improved storage electrodesemployable in target electrode assemblies.

In U.S. patent application Serial No. 630,683 of Harold R. Day, Jr. andPeter Wargo, filed December 26, 1956, entitled Target Electrode Assemblyand assigned to the same assignee as the present application there isdisclosed and claimed a target electrode assembly adapted for use, forexample, in a known type of television camera tube, referred to as animage orthicon. This assembly includes a thin transparent film ofmagnesium oxide and a conductive mesh supported from opposite sides of arelatively rigid glass mesh structure having a large number of closelyspaced openings extending therethrough generally perpendicular to thefilm. This structure is mechanically rigid for minimizing undesirablemechanical vibrations which could result in microphonics or unwantedelectrical signal modulations. Additionally, this structure is adaptedfor maintaining relatively constant electrical characteristics over longperiods of use.

It is considered that the conduction in a direction normal to theopposed faces of the magnesium oxide film is due tothe transport throughthe film of electrons rather than ions, and there is no loss ofavailable electrons in the film after long periods of use correspondingto the decrease in available mobile ions occurring in previouslyemployable glass films. Thus this type of assembly is not subject to theundesirable phenomenon which has been called burn-in and which resultsin a tendency for an after image to be retained on theelectrode for aperiod many times the frame repetition rate, causing the image to besuperimposed uponlater scenes. In operation of the structure disclosedin the above-referenced application Serial No. 630,683, a charge patternis established on the magnesium oxide film in accordance with thesecondary electrons emitted from one surface of the film in response "toimpingement upon the opposite surface thereof of electrons from aphotocathode. Inasmuch as the magnesium oxide film provides a high yieldof secondary electrons, it

results in a sensitive target electrode.

The present invention contemplates an improved target electrode assemblywhich will aiiord all of the advantages obtainable with theabove-described assembly of Day and Wargo without reliance on anintermediate electrode support and which affords substantial'furtheradvantages.

' Accordingly, it is a primary object of my invention to provide a newand improved target electrode assembly 'which maintains its electricalcharacteristics over long periods of use.

Another object of my invention is to provide a new and improved targetelectrode assembly wherein a void is provided between the spacedelectrodes thereof and tent O 2,922,907 Patented Jan. 26, 1960 ice thusis not subject to spurious signals due to charge accumulations on andleakages across elements interposed between the spaced electrodes.

Another object of my invention is to provide a new and improved targetelectrode assembly including new and improved means for avoidingundesirable mechanical vibrations and resulting unwanted electricalsignalmodulations without reliance on support means disposed within thearea defined by the margin of the assembly.

Another-object of my invention is to provide a new and improved storageelectrode adapted for improved image resolution.

Another object of my invention is to provide a new and improved targetelectrode assembly adapted for increased electron transmission and thusadapted for higher sensitivity.

Another object of my invention is to provide a storage electrode whichrequires support only at its periphery and, thus, is capable of highersecondary emission than storage electrodes which require supportstructure across the surface thereof and which are subject to reducedsecondary emission resulting from the adverse effects of foreignmaterials evolving from the support structure.

Another object of my invention is to provide an improved targetelectrode assembly which comprises a reduced number of essentialelements and thus is adapted for reducing. the costs and efforts ofmanufacture.

Another object of my invention is to provide an improved storageelectrode which overcomes the need for utilizing elements formed ofmaterials such as glass which generally add to the difficulties ofevacuating envelope structuresadapted for containing such targetelectrodes.

Further objects and advantages of my invention will become apparent asthe following description proceeds and the features of novelty whichcharacterize my invention will be pointed out with particularity in theclaims annexed to and forming part of this specification.

In carrying out the objects of my invention I provide a target electrodeassembly including a conductive mesh electrode and an annular electrodesupport corresponding generally to the margin of the mesh electrode.Extending tautly across the annular electrode support and supportedsolely thereby is a membrane of polycrystalline homogeneous magnesiumoxide. The membrane is spaced a predetermined distance from the meshelectrode and has a thickness of substantially the same order ofmagnitude as the size of the crystals constituting the membrane. Themembrane is of a mass per unit area which requires for excitation avibrational frequency of such a high magnitude as to be practicallyvibrationless in ordinary target electrode environments. Furthermore,the resonant frequency is such as to be almost undetectable at normalframe and scanning rates. The target electrode is manufactured by firstforming a vaporizable support film on an annular support memberevaporating a magnesium coating on the film and then oxidizing themagnesium and vaporizing the support film, thereby to leave a tautmagnesium oxide membrane supported only by the annular support. Themagnesium oxide membrane is frail or subject to easy breakage anddestruction of the membrane by relative air movement is avoided bymaintaining an air breaking element in closely spaced relation with themembrane during manufacturing and assembly movements thereof. The methodof manufacturing and handling the disclosed target electrode andassembly is disclosed in detail hereinafter and constitutes the subjectmatter of my copending U.S. divisional application Serial No. 838,012entitled, Methods of Manufacturing and Handling Electrodes and TargetElectrode Assemblies filed August 11, 1959, and assigned to the sameassignee as the present invention.

Fig. 2 is an enlarged elevational view in section, illus trating theelectrode assembly, including the target electrode, of the image sectionof tube shown in Fig. 1;

Fig.3 is a perspective view showing the construction;

of the target electrode assembly of the present invention;

Fig. 4 is a perspective fragmentary view, partially in section andgreatly enlarged, showing constructional details of the target electrodeassembly of the present invention.

Fig. is an enlarged fragmentary sectional view illustrating a step inone method of manufacturing the target electrode ofthe presentinvention; and

Fig. 6 is an enlarged fragmentary sectional view illustrating a step inanother method of manufacturing the target electrode of the presentinvention.

As best seen in Fig. 4, the target electrode assembly of the illustratedembodiment of the present invention, includes planar electron-permeableelectrode generally designated 1 and a storage electrode generallydesignated 2 including a transparent magnesium oxide membrane 3supported in spaced relation to the electron-permeable portion of theplanar electrode 1.

The electrode 1 can comprise an electroformed mesh 4 which has beenaluminized on both sides and is mounted on an annular support ring 5.The support ring 5 includes an annular channel 6 in which is received aretaining ring 7 adapted for securely retaining the edge of the mesh 4in the channel 6 and, thus, securing the mesh tothe support ring 5. p iI The magnesium oxide membrane 3 of the storage electrode 2 is supportedsolely by an annular support 8 which is formed to correspond generallyto the outer edge of margin of the mesh supporting ring 5 of theelectrode 1. The support 8 has the membrane 3 directly mounted on theundersurface thereof in Fig. 4 and is suitably secured, as by brazing,to a ring 9 which is of angular cross-section and thus is adapted forrigidizing the ring 8. An annular spacer 10 interposed between theelements 5 and 8 determines the spacing between the mesh 4 and themembrane 3. Thus, the mesh 4 and the membrane 3 are adapted for beingmaintained in a spaced relation or are separated by a void which extendscompletely across the corre sponding areas of these elements. Preferablythe spacing can befrom approximately .5 to 150 mils. For monochrometelevision purposes a spacing of 2.5 to 3 mils is highly satisfactory,while a spacing of approximately .5 'mil works well for color televisioncamera tubes. The spacing between the mesh and membrane determines thetime constant of the assembly and it will be understood from theforegoing that this time constant can be varied by varying the spacingwithin the limits of the range noted.

4 water at the outer portion of the film, is raised pickup the film onthe surface of the ring.

After the film has dried completely, the ring is placed in an evaporatorand a thin coating of magnesium shown at 12 in Fig. 5 is evaporated onthe plastic film to a desired thickness. The thickness of the magnesiumthus evaporated on the film is determined by the desired megently to ichanical and electrical characteristics of the storage electrode. In theparticular embodiment illustrated, the film of magnesium isapproximately 500 angstroms thick.

Thereafter, the electron-permeable electrode 1 and the just-describedtarget electrode structure are assembled in the manner shown in Fig. 5and the whole assembly is placed in an oven and baked out in air,starting at a temperature of about 170 C. and terminating at about 400centigrade for a period in the order of approximately five hours. Thisbaking decomposes and vaporizes the nitrocellulose film which disappearscompletely and also is efiective for converting the magnesium to anoxide for thus forming a smooth, taut, transparent, colorless magnesiumoxide film or membrane. During the baking operation and subsequentlyduring handling of the as sembly the magnesium oxide membrane, which isvery flimsy or frail, is prevented from being destroyed by air movementrelative thereto by the mesh 4 of the electrode 1 which serves as an airbreak or air baffling element. In view of the fact that the mesh 4 isincluded in the assembly which is subjected to the air bake it isessential that the mesh 4 be capable of withstanding the air bakingoperation without oxidizing.

With a magnesium oxide membrane thickness of 500 angstroms, the timeconstant of the storage electrode structure issuitable for a repetitionrate of 30 frames per second used in television. The time constantincreases as the thickness of the magnesium oxide membrane increases andinformation storage may be realized with magnesium oxide films of athickness in the order of several thousands of angstroms.

The substance of the magnesium oxide membrane 3 3 by first dropping ontothe surface of a pan of water a small quantity of nitrocellulosedissolved in a suitable organic solvent such as amyl acetate. Thissolution spreads out into a thin film due to surface tension and thesolvent evaporates, leaving a plastic film on the 'surface of the water.Thereafter, the membrane support ring 8, which has been placed in thewater either prior to the formationofthe film or which is immersedin'the In membrane 3 the conduction is electronic. That is, there is noreliance on ions which in prior art glass membranes has a tendency tobecome depleted and, therefore, the target conductivity in the presentstructure is stable and burn-in is eliminated. By the use of X-raydefraction patterns I have learned that the magnesium oxide membraneobtained by the above-described method is homogeneous andpolycrystalline and that the crystal size is about 300 angstroms. Thus,in a membrane having a thickness of the order of 500 angstroms themembrane thicknesses is of substantially the same order of magnitude asthe size of crystals constituting the membrane; and therefore,conductivity through the film by grain boundary conduction orbombardment induced conductivity is enhanced. This type of conductionenables the utilization of magnesium oxide as a membrane material eventhough it has a much higher resistivity than glass formerly used. Thishigh resistivity results in extremely low lateral leakage which can berelied upon to improve image resolution.

In considering the substantially large area of the magnesium oxidemembrane 3, it might be felt that the assembly would be subject tomicrophonics due to drum head vibrations of the membrane. However, inthe assembly of the present invention microphonics are substantiallyreduced due to the fact that the mass per unit area of the membrane 3 isabout times less than glass membranes formerly employed. Thus, thevibrational frequency of the membrane 3 is approximately 10 times ashigh as for a glass membrane when those types of membranes are under thesame tension. Additionally, the membrane 3 can be placed under greatertension per unit cross section than can prior art glass membranes. Thevibrational frequency of the membrane 3 is more diflicult to excite andeven if excited image variations resulting therefrom are not generallydiscernible at normal seaming rates and thus are not generallyobjectionable.

For all practical purposes the membrane 3 might be consideredvibrationless in ordinary target electrode environ ments.

Thus, it will be seen that the assembly of Figs. 3 and 4 is adapted,since it operates by means of electron conduction rather than ionconduction for maintaining its electrical characteristics over longperiods of use. Additionally, the membrane 3 is effective for, avoidingthe undesirable mechanical vibrations-and resultant microphonics withoutreliance on an intermediate support such as the glass mesh disclosed inapplication Serial No. 630,683.. This has the desired effect ofeliminating the need for the glass mesh which is currently asubstantially expensive and difficult to manufacture element.Additionally, by providing a structure including a void between the meshand membrane 1 have reduced the possibilities of spurious signals due tocharge accumulations and leakages across portions of material extendingbetween the electrodes. Due to the void between the elements electrontransmission through the assembly is increased, thereby to affordimproved image resolution. Also, increased electron transmission adaptsthe assembly for a higher sensitivity. Further, in structures utilizinga membrane support extending across the surface of the membranesecondary emission can be reduced substantially due to foreign matterevolving from the material of the support. Applicants membrane 3 doesnot require this type of support and, thus, is adapted fordesirablygreater secondary emission. Still further, glass is noted as amaterial which desorbs vapor upon heating and the elimination of glassfrom the target assembly serves not only to eliminate the need for arelatively expensive element and to simplify the structure and effort ofmanufacture, but also serves to remove from a tube another element whichmight constitute a source of difficulty in evacuating an envelopecontaining the assembly.

Alternatively, the assembly of Fig. 3 can be obtained by forming avaporizable film 11 on the annular ring 8, then coating the film withmagnesium in the manner illustrated at 12in Fig. 6, and then air bakingthis assembly whilesupported, for example, on a plurality ofcircumferentially spaced upstanding elements 13. Thus, the vaporizablefilm 11 is decomposed and the magnesium coating. 12 is converted to amagnesium oxide membrane 3 which extendscompletely across the member 8and is solelysupported thereby. When the storageelectrode is formed inthis manner it is necessary to transport it for assembly with theelectron permeable electrode 1 and it is necessary to accomplish thiswithout fracture or destruotion of the membrane 3. To accomplish this Iplace a plate 14 over the target electrode 2 and in engagement with therim of the element 9 and by concurrently grasping the contiguous edgesof these elements it is possible to lift both and transport the targetelectrode to a position where it can be assembled to the electronpermeable electrode 1 with the element 14 acting as an air break or airbafliing means, thus to avoid fracture or destruction of the membrane bymovement of air relative to the support 8 therefor. After the targetelectrode is assembled to provide the target electrode-electronpermeable electrode assembly, the mesh 4 of the electrode 1 can serve toavoid destruction of the membrane 3 during movement of the targetelectrode assembly for mounting in a camera tube.

As best seen in Figs. 2, 3, and 4 the mesh support ring 5 has securedthereto a plurality of angularly spaced brackets 15 which carryrotatable resilient tabs 16. The tabs 16 are adapted for being movedinto the radially inwardly projecting positions thereof illustrated inFigs. 3 and 4 wherein they are effective for retaining the targetelectrode 2 in assembled relation with respect to the electrode 1.Additionally, the support 5 carries a plurality of angularly spaced andlaterally slotted mounting tabs 17 which are adapted for mounting theassembly in a camera tube in the manner illustrated in Fig. 2. In thearrangement of Fig. 2 the target electrode assembly is supportedtransversely in a cylindrical target supporting electrode 20 by means ofthe slotted mounting tabs 17 being fitted about suitable holding bolts21 carried byan inturned flange 22 formed on the cylinder 20. Thus, thetarget electrode assembly is supported opposite the opening in acylindrical flange 23 comprising part of the target assembly supportingelectrode 20. The latter electrodeforrns part of the assembly includingan accelerating grid electrode 24 and a decelerating grid electrode 25.These three electrodes are sup-. ported in longitudinally spaced coaxialrelation by suitable ceramic rods or stalks 26 spaced around thecircumference of the electrodes and held thereto by suitable straps 27.This assembly is supported in the enlarged image section of the tube'shown schematically in Fig. 1 with the accelerating grid electrode 24spaced slightly from a photocathode 28 which provides a source ofphotoelectrons. The photoelectrons are accelerated toward the targetelectrode assembly to establish a charge pattern thereon in accordancewith the image falling on the photocathode. At the opposite end of thetube is the electron gun and electron multiplier structure which areconcentrically arranged. The gun, which provides the scanning beam, isshown merely as a hollow cylindrical grid electrode 30, having a smallaperture 31 in the order of .002 inch in diameter in the end wallthereof, for producing a thin scanning beam. The outer surface of thisend wall surrounding the aperture also provides the first dynode of theelectron multiplier, as will be described in more detail hereinafter. Acylindrical electrode which may be formed as a metallic coating 32 onthe neck of the tube provides for focusing the beam and the fieldcontrolling electrode'25, usually designated a decelerating electrode.As will be'readily appreciated by those skilled in the art, the entirecamera tubeis subjected to an essentially homogeneous longitudinalcollimating magnetic field. This field may have a strength of -75 gauss,for example. Electrons from the scanning beam are collected inaccordance with the charge or potential pattern established on thetarget electrode so that returned electrons, which are the forward beamelectrons minus those collected, vary with the charge pattern on thetarget electrode 3. These electrons do not reenter the aperture 31 butinstead strike the plate surrounding the aperture, which is a highsecondary'emitter so that there results a multiplication of theelectrons emitted compared with those returned from the storageelectrode.

A generally cylindrical focusing electrode 33 for the electronmultiplier section of the tube is supported at the end of the gunelectrode 30 intermediate that electrode and the beam focusing gridelectrode 32.

Several stages of electron multiplication are provided by electrodes34-37 inclusive and the amplified electron current is collected by theanode 38 of the electron multiplier to produce a signal across theresistor 39 which varies in accordance With the charge pattern on themembrane 3. In Fig. l of the drawing, suitable direct current operatingvoltages for the various electrodes have been indicated. These voltagesare relative to the cathode and may vary appreciably from the valuesgiven.

When the target electrode is scanned by an electron beam, the variationsin beam current collected by the anode 3S reproduces point-by-point anelectrical signal varying in accordance with a charge pattern on thetarget electrode. The time constant of the membrane 3 determines theframe speed on which the device operates, hence it is essential that theresidual charges from one frame to another be so small as not tointerfere with the production of an electrical signal indicative of theimage falling on the photocathode in any particular frame.

With the magnesium oxide membrane 3, the electrical characteristicsremain relatively constant over extended life of the membrane. Theundesirable burn-in phenomenon resulting from what is generallyunderstood to be a depletion of the mobile ions in glass membranes isnon-existent. The magnesium oxide membrane.3 provides a target which isavailable on both faces for the impingement of the electron. Also, thestructure of the present invention provides for a relativelyvibrationless target electrode which is substantially free of,undesirable microphonics due to target electrode movement. Additionally,in the present structure no intermediate electrode support means isrequired which affords improved resolution, higher sensitivity, highersecondary emission,free dom from certain spurious signals, andreductions in costs and efforts in manufacturing the electrode assemblyas well as an evacuated device incorporating same.

While I have shown and described a particular embodiment of myinvention, I do not desire my invention to be limited to the particularform shown and described, and I intend by the appended claims to coverall modifications within the scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A storage electrode comprising an annular support member having adimension comparable to that of a target electrode assembly, and astorage membrane consisting of homogeneous magnesium oxide extendingacross said annular support member and solely supported thereby.

2. A storage electrode comprising an annular support member comparablein size to a target electrode assembly, and a storage membrane ofhomogeneous polycrystalline magnesium oxide having a thickness ofsubstantially the same order of magnitude as the size of crystalsconstituting said membrane, said membrane extending across said supportmember.

3. A storage electrode comprising an annular support member the size ofa target electrode assembly, and a storage membrane of homogeneouscrystalline magnesium oxide having a thickness of approximately 500angstroms and constituted of crystals having a size of approximately 300angstroms, whereby conductivity through said member is facilitated andlateral leakage is minimized, said storage membrane extending acrosssaid support member.

4. A target electrode assembly for establishing a pointby-point chargepattern in accordance with information to be converted to an electricalsignal by scanning said target electrode with an electron beam, saidassembly comprising a planar electron permeable electrode and ahomogeneous polycrystalline magnesium oxide -mem brane corresponding inarea to said planar electrode, said membrane being solely supported atthe periphery theredfand separated from said planarelectrode by a voidextending completely across the corresponding areas thereof.

-"5. A target electrode assembly for'establishing a pointby-pqint chargepattern in accordance .with information to be converted to an electricalsignal by scanning saidtarget electrode with an electron beam, ,saidassembly comprising a mesh electrode, a. homogeneous polycrystallinemagnesium oxide membrane corresponding in area to said mesh electrode,said membrane being supported only at the periphery thereof and beingspaced from said mesh 'electrode'by a spacing of between approximately.5 to mils.

6. A target electrode assembly for establishing a point: by-point chargepattern in accordance with information to be converted to an electricalsignal by scanning said target electrode with an electron beam, saidassembly comprising a mesh electrode, an annular support membercorresponding generally to the margin of said mesh electrode, apolycrystalline storage member extending tautly across said supportmember and being solely supported thereby, said membrane being separatedfrom said mesh electrode by a void of approximately .5 to 150 milsextending across the corresponding areas of said mesh and membrane andhaving a thickness of substantially the same order of magnitude as thesize of crystals constituting said membrane.

References Cited in the file of this patent UNITED STATES PATENTS2,189,340 Donal Feb. 6, 1940 2,335,705 Smith Nov. 30, 1943 2,544,753Graham Mar. 13, 1951 2,544,754 Townes Mar. 13, 1951 2,563,488 Rose Aug.7, 1951 2,582,843 Moore Jan. 15, 1952 2,743,150 Rudy Apr. 24, 19562,776,387 Pensak Jan. 1, 1957 2,795,840 Salecker June 18, 1957 2,819,419De Lano et al. Jan. 7, 1958 2,822,493 Harsh Feb. 4, 1958

1. A STORAGE ELECTRODE COMPRISING AN ANNULAR SUPPORT MEMBER HAVING ADIMENSION COMPARABLE TO THAT OF A TARGET ELECTRODE ASSEMBLY, AND ASTORAGE MEMBRANE CONSISTING OF HOMOGENEOUS MAGNESIUM OXIDE EXTENDINGACROSS SAID ANNULAR SUPPORT MEMBER AND SOLELY SUPPORTED THEREBY.